1. Field of the Invention
The present invention relates to an optical scanning system for printers and a method for adjusting the starting point of image scanning, and more particularly, to an optical scanning system for printers, in which a defective area is detected at the edge of a photoreceptor web and a starting point of line image scanning is adjusted according to the detection result.
2. Description of the Related Art
FIG. 1 shows a general electrophotographic color printer. Referring to FIG. 1, a reset unit 15, optical scanning units 30, photodetectors 18, developer units 16, a drying unit 19 and a transfer unit 20 are positioned around a photoreceptor web 14 circulating around a plurality of rollers 11, 12 and 13.
During the printing operation, the optical scanning units 30 scan light onto the photoreceptor web 14 circulating through the reset unit 15 and an electrostatic latent image is formed on the photoreceptor web 14 by the scanned light. The electrostatic latent image is developed by using a developer supplied from the developer units 16. A color image is formed on the photoreceptor web 14 by the optical scanning units 30 which emit lights for each of the respective colors, and by the developer units 16 which develop the electrostatic latent image with the corresponding colors. The color image formed on the photoreceptor web 14 is first transferred by a transfer roller 21 which rotates partially engaged with the photoreceptor web 14, passing through the drying unit 19 with continuous movement of the photoreceptor web 114. In addition, the image transferred onto the transfer roller 21 is again transferred on a paper 23 with the rotation of the transfer roller and a fixing roller 22.
FIG. 2 shows a conventional optical scanning system adopted in the printer shown in FIG. 1. The optical scanning system comprises an optical scanning unit 30, a photodetector 18 and an optical scanning controller 35. The optical scanning unit 30 scanning light onto the photoreceptor web 14 in a main scanning direction, parallel to the width of the photoreceptor web 14. The photoreceptor web 14 moves in a direction indicated by an arrow A. The optical scanning unit 30 comprises a light source 31, a rotary polygonal mirror 32 and a lens unit 33. Here, a direction perpendicular to the movement direction of the photoreceptor web 14, and parallel to the width of the photoreceptor web, is referred to as a main scanning direction. Further, a direction perpendicular to the main scanning direction, and in the movement direction of the photoreceptor web 14 is referred to as a sub-scanning direction.
The photodetector 18 is installed so as to detect the light emitted toward the edge of the photoreceptor web 14 by the optical scanning units 30. The optical scanning controller 35 synchronizes initiation of scanning light corresponding to image information, onto an image writing area D set in the photoreceptor web 14, according to a pulse signal output from the photodetector 18 in response to the reception of light. That is, the optical scanning controller 35 determines an end point of the reception of the pulse signal from the photodetector 18 as the point in time at which the light from the optical scanning units 30 reaches the edge of the photoreceptor web 14. The optical scanning controller 35 controls the optical scanning units 30 such that the emission of light, corresponding to the image information from a light source 31 starts after a predetermined delay time from the end point of the reception of the pulse signal. This delay time allows the scan light from the optical scanning unit 30 to be properly concentrated inside the writing area D of the photoreceptor web 14.
However, because the light source 31 emits light corresponding to the image information after a predetermined delay time, the scanning of image information may deviate from the writing area D when the edge of the photoreceptor web 14 has a defect at its edge, thereby causing error in writing of the image.
To solve the above problems, it is an object of the present invention to provide an optical scanning system for a printer, capable of suppressing errors in writing images by compensating for a defective area of an edge of a photoreceptor web through adjustment of a line scanning starting point, and a method for adjusting the starting point of line scanning onto the photoreceptor web.
According to an aspect of the present invention, there is an optical scanning system for a printer, comprising: an optical scanning unit for scanning light onto a circulating photoreceptor web; a photodetector for receiving scanning light emitted from the optical scanning unit over the edge of the photoreceptor web and outputting pulse signals corresponding to the detected light; a defective edge area detector for comparing the photodetector pulse signals in sequence to determine whether the edge of the photoreceptor web has a defect, and outputting an edge defect signal if a defective area is detected; an estimated pulse width calculator for outputting a predetermined estimated pulse width, and updating the estimated pulse width for the next line image scanning, wherein the estimated pulse width is calculated from the signal from the defective edge area detector, the predetermined estimated pulse width, and the pulse signal from the photodetector; a line scanning synchronous signal generator for generating a line scanning synchronous signal in synchronicity with the predetermined estimated pulse width; and an optical scanning unit driver for driving the optical scanning unit such that the optical scanning unit emits the light onto image information synchronized with the line scanning synchronous signal.
Preferably, the optical scanning system further comprises a belt weaving speed calculator for calculating the moving rate of the edge of the photoreceptor web in a light scanning direction. The moving rate is calculated from the pulse signal of the photodetector or information about the estimated pulse width from the estimated pulse width calculator until the edge defect signal is received. The belt weaving speed calculator also calculates a correction value to correct the estimated pulse width, from the calculated moving rate. The estimated pulse width calculator adds the correction value to the predetermined estimated pulse width, and updates the next estimated pulse width with the addition result when the edge defect signal is received.
According to another aspect of the present invention, there is provided a method of adjusting a starting point of image scanning in an optical scanning system for a printer, the optical scanning system comprising an optical scanning unit installed to scan light onto a circulating photoreceptor web, and a photodetector capable of receiving scanning light emitted from the optical scanning unit that shines over the edge of the photoreceptor web, the method comprising the steps of: (a) counting the width of pulses output from the photodetector in response to the reception of light by the photodetector; (b) generating a line scanning synchronous signal for synchronizing initiation of image scanning in the optical scanning unit, the synchronous signal generated after a duration of time equivalent to a predetermined estimated pulse width plus a predetermined reference value, from a point in time when the pulse is generated by the photodetector; (c) determining whether the edge is of the photoreceptor web is a defective area by comparing the difference between the pulse width counted in the step (a) and the pulse width output from the photodetector just before the step (a); (d) if the edge of the photoreceptor web is determined to have a defective area in the step (c), adjusting the predetermined estimated pulse width and updating the next estimated pulse width with the adjusted pulse width; and (e) if the edge of the photoreceptor web is determined not to have a defective area in the step (c), comparing the predetermined estimated pulse width and the width of pulse counted in the step (a) to determine the next estimated pulse width.
Preferably, the step (d) calculates the moving rate of the edge of the photoreceptor web in a light scanning direction using the pulse signals from the photodetector or information about the estimated pulse width having been updated in order. A correction value is calculated according to the moving rate, the correction value is then added to the predetermined estimated pulse width, and the next estimated pulse width is updated with the result of addition.
Preferably, step (e) comprises the steps of: (e1) if the width of pulses counted in the step (a) is smaller than the predetermined estimated pulse width, the next estimated pulse width is updated with a value obtained by subtracting a predetermined reference value from the predetermined estimated pulse width; (e2) if the width of pulses counted in the step (a) is greater than the predetermined estimated pulse width, the next estimated pulse width is updated with a value obtained by adding a predetermined reference value to the predetermined estimated pulse width; and (e3) if the width of pulse counted in the step (a) is equal to the predetermined estimated pulse width, the next estimated pulse width is updated with the predetermined estimated pulse alone.